1. Field of the Invention
The present invention relates to a metal coated carbon brush for electric motor.
2. Description of the Related Art
With reference to FIG. 1, recently on electric motor 2 having a carbon brush 1 (hereinafter referred to as brush) has been getting compact and required to have a large capacity. Therefore, as the carbon brush 1, small size, lower resistance loss of electric conductivity (hereinafter referred to as resistance loss) and lower wear has been required.
Conventionally, as the carbon brush used for slip ring or low voltage electric motor, a metallic carbon brush which consists of graphite powder mixed with metal powder and sintered has often been used. However, as increasing the metal content reduces the resistance loss of carbon brush, there are some problems such as inferior lubrication, anti-arc property, and increasing wear.
Also, as an AC commutator motor, there is a problem that a commutation gets worse and the wear increases if low resistance material is used to decrease the resistance loss.
To the contrary, in the case of using high resistance material and applying greater current to a brush, the brush temperature increases due to resistant heating. Although the brush is usually produced by embedded a lead wire with copper powder or the like and compressed to join with carbon brush body. As current is applied, there is a problem that the brush temperature increases and copper powder of caulked part is oxidized and current flow gets worse and at last electric motor stops.
Also, among the AC commutator motor, the electric motor required for higher revolution such as for an electric cleaner, there is a resin bonded type carbon brush, which consists of graphite powder and resin binder and combines and cured, due to the requirement of excellent commutation at the time of high number of revolution during long life time. However, in case of resin bonded type brush, the brush temperature is increased due to the high resistance, under the high current density, and resulting in degradation of resin used as binder.
To solve these problems, a carbon brush which surface is coated with metal having a good electric conductivity to decrease the electric resistance of whole carbon brush which includes the carbon as a component is well known (For example, patent reference 1 is referred).
Japanese unexamined (Kokai) patent publication Hei 5-182733 is shown for example.
However, since it was difficult to coat the metal with same thickness on the surface of carbonaceous material, due to the variation of the thickness, in some case resulting in uneven coloring on the surface of the metal. Therefore, brush users sometimes feel uncomfortable. As the uneven coloring also causes the oxidation of the metal, it cannot maintain the good electric resistance.
Accordingly, the purpose of the present invention is to provide a metal coated carbon brush having a uniform thickness of metal coating and to prevent the uneven coloring.